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    <script>
        /*
     以下这种异步回调函数嵌套异步回调函数的写法被称作为回调地狱
     回调地狱耦合度过高,不方便维护和阅读
   */

        console.log('请求A数据...');
        setTimeout(() => {
            console.log('A数据请求成功');
            console.log('请求B数据...');
            setTimeout(() => {
                console.log('B数据请求成功');
                console.log('请求C数据...');
                setTimeout(() => {
                    console.log('C数据请求成功');
                },2000)
            }, 1500)
        }, 3000)

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